A fuel cell generates electric power by a chemical reaction between hydrogen as fuel and air containing oxygen as oxidant. The fuel cell system may use a circulation mechanism to circulate unreacted fuel gas (hydrogen gas) discharged from the fuel cell. In this case, not only the hydrogen gas but also nitrogen and water are in a hydrogen gas circulation conduit. Thus, impurities such as nitrogen gas and water are concentrated as the operation continues. Hence, the fuel cell system often has an exhaust mechanism that discharges gas and a part of water from a hydrogen gas exhaust conduit as necessary to achieve a hydrogen partial pressure needed for power generation.
At low temperature starting such as starting below a freezing point, water left in the fuel cell or the like may freeze and clog a part of the hydrogen gas exhaust conduit. In this case, when a hydrogen gas conduit is a circulation conduit, the hydrogen gas is reduced due to the chemical reaction with oxygen in the fuel cell, while unreacted nitrogen gas is not discharged from the hydrogen gas exhaust conduit, and the nitrogen gas is accumulated and concentrated in the clogged hydrogen gas exhaust conduit. As a result, the hydrogen partial pressure decreases, and the power generation voltage of the fuel cell decreases. When the power generation voltage decreases to a negative voltage, the fuel cell may deteriorate. International Publication No. 2011/021301 discloses an art that stops a pump used to circulate hydrogen when a hydrogen gas conduit in the fuel cell is clogged. The stopping of the pump stops the circulation of nitrogen, prevents the nitrogen from flowing into the fuel cell, and reduces the decrease in hydrogen partial pressure.